Methanol is a simple alcohol which is used as a reagent in many manufacturing processes and as a fuel.
The demand for green sources of methanol has increased recently, due in part to increasing prices of other energy sources, a demand for non-fossil fuels and a demand for methanol as a component in other processes. This increase in demand has lead to a renewed interest in the purification of raw methanol.
During the kraft or sulfate process of pulp production in pulp mills, methanol is obtained as a by-product. The obtained raw methanol is contaminated with impurities, such as sulfurous organic compounds, ethanol, ammonia, as well as turpentine (mixture of various terpenes). Kraft pulping consists of the digestion of wood chips at high temperature and pressure in diluted alkaline liquor containing sodium hydroxide and sodium sulfide as active ingredients. The digestion disrupts the cellular structure and causes the dissolution of lignin, other chemical products contained in the wood and hemi-cellulose. Thus, the cellulose fiber dispersed in the spent liquor from the digestion may be isolated by filtration. The reaming liquor, known as black liquor, is further evaporated and calcinated for the recovery of salts and alkalis, which return to the kraft pulping process.
Raw methanol is obtained during the process by the demethylation of methoxyl groups found mainly on the lignin polymer. Raw methanol is primarily obtained in the condensates from the evaporation of the black liquor. However, raw methanol is also obtained at other points in the pulping process, such as in condensates and vapors from the digestion phase.
During pulp production from pine and spruce around 6 kg of raw methanol is obtained per tonne of pulp (theoretically up to about 9.5 kg could be obtained). The methanol is obtained in the condensates from black liquor evaporation and condensates from the digestion system and enriched by distillation. The methanol content in this enriched stream is typically fairly high (>75 wt. %) although being contaminated. It is further possible to extract more methanol than 6 kg per tonne as there are more sub-processes in a pulp mill where a cleavage of methyl groups results in the formation of methanol, such as oxygen bleaching/bleaching/evaporation. However, this is not performed today as the raw methanol has little to no economic value.
The 6 kg or more of raw methanol could be considered a small amount of energy from an energy usage perspective but it is indirectly important as it is a green methanol with an effective carbon footprint of 0.
The primary reason that the raw methanol is not used in other processes is that it is usually deemed impossible to transport off site due primarily to odor problems, as the raw methanol is contaminated with sulfurous compounds, e.g. H2S, methyl mercaptan, dimethyl sulfide, but also less volatile sulfurous compounds. When other dangers are considered, such as high H2S concentrations and toxicity, basically all pulp mills have opted to destroy the methanol by combustion using completely closed systems directly after the extraction of methanol from the evaporation system.
There are existing processes for purifying raw methanol. Such a process is described in FI52710 by Suokas for Kemi OY. In the disclosed process a stream of raw methanol is first acidified causing the precipitation of ammonium salts, subsequently the raw methanol is distilled, whereby any volatile sulfurous compounds, e.g. H2S, methyl mercaptan, and dimethyl sulfide, are removed while methanol remains in the bottom fraction. The raw methanol is then treated with an oxidizing agent to increase the boiling point of any remaining dissolved sulfurous compounds. Subsequently, methanol is separated from the water phase, comprising high boiling, oxidized sulfurous compounds, by distillation or steam stripping to provide methanol with a low content of sulfurous compounds.
U.S. Pat. No. 8,440,159 to Metso Power AB relates to a similar process as the one disclosed in FI52710. In the Metso process the acidified raw methanol mixture is heated whilst maintaining a concentration of ammonium salts below the concentration at which they precipitate. The raw methanol is evaporated from the acidified mixture and subsequently treated as in FI52710.
Both these processes require a significant amount of chemical reagents, e.g. oxidating agents, which are costly to both procure and dispose of. They furthermore require extensive capital construction costs.
The relatively low volume available at each production facility combined with the relatively high construction and reagent costs implies that it is unprofitable to install any of the existing technology solutions. Accordingly, methanol is still just burned of at the pulp mill to generate heat.
Nor is there any possibility to transport raw methanol to a central processing facility to achieve economies of scale. Such a system would need to be designed to process the entire Scandinavian region's overall volume of raw methanol to produce enough methanol, to account for the construction costs for such a plant.
As a result of the unprofitability of installing existing purification systems, it is still deemed to be more cost-effective to simply burn the methanol.
More efficient methods of purification are thus required for it to be possible to purify raw methanol in a cost-effective manner, whereby green methanol for production of e.g. methyl esters of fatty acids of plant origin (FAME), i.e. biodiesel, could be produced as a by-product in pulp production.